Electrosurgical and in particular electrothermal sclerosing of pathologically altered tissue, hereinafter referred to for brevity as tissue, is a method which is known in medicine. That method is of particular interest for the therapy of organ tumors, in particular liver tumors. For the sclerosing procedure, one or more electrodes are placed in the tissue to be sclerosed, that is to say the tumor tissue, or in the immediate proximity thereof, and an alternating current is caused to flow between the electrodes or an electrode and a neutral electrode fixed externally to the body. If the current flows between the electrode and the neutral electrode (possibly also between a plurality of electrodes and one or more neutral electrodes), that is referred to as a monopolar electrode arrangement. If in contrast the current flows between the electrodes themselves disposed in the tissue (in that case there must be at least two electrodes in the tissue), that is to referred to as a bipolar arrangement. An arrangement is referred to as a multipolar arrangement when more than two electrodes between which alternating current flows are present in the tissue.
The electrodes intended for placement in the tissue are generally in the form of electrode needles. They have an electrically conducting cylindrical shaft which, with the exception of one or more distal regions, the so-called active regions of the electrode or, for brevity, active electrodes, is electrically insulated relative to the ambient tissue. In contrast the active electrodes are electrically conductively connected to the body tissue. The active electrodes are optionally also equipped with integrated thermosensors. In especial embodiments further active electrodes can be mechanically extended at the distal end of the shaft in order to enlarge the volume of tissue to which therapy can be applied.
A flow of current is induced between the active electrodes and the neutral electrode or electrodes by means of a high frequency generator, in the monopolar arrangement. In the alternative bipolar arrangement, it is possible to eliminate the neutral electrodes. In that case the circuit is closed by way of a further active electrode, in which respect the required active electrodes can be arranged in a coaxial configuration in mutually insulated relationship on the electrode needle or on two separate electrode needles.
The ohmic resistance of the tissue, which is a part of the complex tissue impedance, causes the alternating current applied by way of the electrodes to be converted into joulean heat. At temperatures of between 50 and 100° C., massive denaturisation of the body-specific proteins (coagulation) occurs and consequently the tissue area in question is caused to die off. By virtue of the high current density at the active electrodes heating takes place predominantly in the region of those electrodes so that local thermal tumor destruction is possible.
An apparatus for and a method of electrothermal sclerosing of pathological tissue is disclosed by way of example in U.S. Pat. No. 5,630,426.
What is crucial for effective and in particular reliable therapy is the production of a thermal destruction zone which is optimally adapted to the extent of the pathological tissue, that is to say the tumor tissue.